Multicolor printers generate images which are composed of a plurality of different single-color images. The quality of the final multicolor image depends on the registration accuracy of the single-color images. With the increasing resolution of modern printers the registration accuracy has become an issue of interest.
Different multicolor printer types are known. Ink-jet printers have at least one print head from which droplets of ink are directed towards a recording medium. Within the print head the ink is contained in a plurality of channels. Pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels. These pulses are generated e.g. by thermal components in thermal ink-jet print heads or by piezo-electric elements in drop-on-demand print heads. Ink-jet printers of the carriage type have a print head for each color. The print heads are mounted on a reciprocating carriage. Full width or page width ink-jet printers have, for each color, an array of nozzles extending across the full width of the print medium which is moved past the nozzle arrays. Each nozzle array is part of a print station which produces one single-color image or a part of it. Each print station provides its own color image or pattern on the recording medium as it moves past the print stations. Each pattern is formed of a plurality of closely spaced ink dots, wherein single-color ink dot patterns are superimposed to form a multicolor pattern which represents the multicolor image. The print medium may be paper or any other suitable substrate to which the ink adheres.
In known color xerographic systems, instead of the nozzle arrays, a plurality of LED print bars are provided adjacent to a photoreceptive surface. The print bars are selectively energized to create successive charge patterns, one for each color. Each LED print bar is associated with a development system, which develops a latent image of the last charge pattern or exposure without disturbing previously developed images. The fully developed color image is then transferred to an output sheet, e.g. paper or the like.
To register single-color image patterns for forming a multicolor image, encoder arrangements are utilized which determine the advance of the recording medium during the print process. Optical encoder systems are known in which an optical sensor is responsive to encoder marks.
In page-width printers the recording medium is, for example, moved by a conveying belt which is driven by rollers or pulleys. The movement of the belt with the recording medium may be detected by a single rotary encoder which is mounted on one of the rollers or pulleys. The advance of the belt is controlled by advance information represented by the rotary encoder signals. It is also known to place the encoder marks on the belt.
U.S. Pat. No. 6,155,669 discloses an ink-jet printer with several print stations and a conveying belt with encoding marks. Each print station has its own optical reader responsive to the encoding marks to generate its own position signal.